The invention is directed to a method to measure the ability of a flowing powder to electrostatically charge by allowing the powder to flow under the influence of gravity and measuring the electrostatic charge of the powder. The invention is also directed to a measurement device to measure the ability of a flowing powder to electrostatically charge.
It is well known in the art that electrostatic charges are created inside a powder during a flow. This apparition of electric charges is due to the triboelectric effect, which is a charge exchange at the contact between two solids. During the flow of a powder inside a device, such as a mixer, a silo or a conveyor, the triboelectric effect takes place at the contact between the solid powder particles and at the contact surface between these particles and the device. In particular, triboelectric effect takes place when two particles are sliding on each other or when a particle is sliding on a surface or when a particle impacts an other particle or when a particle impacts a surface. Therefore the characteristics of the powder and the properties of the material used to build the device are important parameters.
The apparition of electric charges inside a powder can produce sparks and cause explosion hazards. In addition, the electrostatic charging during powder particle flow induces agglomeration and segregation which may cause dosage problems. In some applications, the apparition of electric charges inside a powder is an advantage. The best-known application based on powder electrification is the electrophotography. The powder coating process is also based on the adhesion of charged powder particles onto a surface. In granular mixtures, the dependence of the electrification on the powder particles is interesting in separation techniques.
In order to optimize the processes used in these applications, a method and measurement devices are used to measure the ability of a powder to be electrostatically charged. Such methods and devices may be advantageously used to test the effect with respect to the electrostatic charge of an antistatic additive, the effect of the different powder characteristics and/or the effect of the material of the above referred to device. A review article by S. Matsusaka et al in Chemical Engineering Science 65: 5781-5807, 2010 describe different methods and devices to measure the ability of a powder to be electrostatically charged. These methods include the so-called cascade method on an inclined plate, the flow method on a vibrated plate, the fluidized bed method, the centrifugal method and the cyclonic method.
In an article of Peter Ireland (Powder Technology 198 (2010) 189-198) a method is described wherein the accumulated electrostatic charge of a powder and the mechanical attributes of a particulate flow are investigated using a flat stainless steel slide. The powder is feed on the top of the inclined plane by a funnel filled with the powder. With the inclined plane technique of this article, the powder is spreading over the whole plane and the grains are sliding, rolling and bouncing on the plane.
The above methods have the disadvantage that they are difficult to use in practice for all type of particles or grains and furthermore give fluctuating results. In particular, highly cohesive powders will not flow regularly through the aperture of the feeding funnel used in the inclined plane technique. On the other hand, the grains submitted to multiple bounce with random reorientation may also fall outside the plane in the inclined plane technique. Finally, these methods do not concern dense flows. Therefore, the results obtained with these methods are difficult to compare with triboelectric charging effects observed in devices wherein the powder is present in a more dense composition.